The primary function of the alimentary or gastrointestinal (GI) tract is to provide the body with a balanced supply of water, electrolytes and nutrients. In order for this to be achieved, food must be moved along the GI tract at an appropriate rate for digestion, absorption and secretion to take place. Food is normally transported through the GI tract in a well-coordinated manner by propulsive movements which are mediated by clusters of smooth muscle contractions known as migrating myoelectric complexes, in a process commonly referred to as peristalsis.
Defects in the normal motility pattern can lead to the development of chronic, painful and debilitating disorders. For example, an incompetent or weak lower esophageal sphincter may result in frequent reflux of ingested food from the stomach into the esophagus which may lead to esophagitis. Prokinetic agents (also called motility-enhancing agents) are useful in treating reflux esophagitis because they (a) increase the pressure of the lower esophageal sphincter, thereby inhibiting reflux; (b) increase the force of esophageal peristalsis to facilitate clearance of food from the esophagus into the stomach; and (c) increase gastric emptying, thereby further decreasing the mass available for reflux.
There is a need, however, for improved prokinetic agents in the treatment of this disorder. Presently used cholinergic drugs such as bethanechol and dopamine receptor blocking agents such as metoclopramide may exhibit serious disadvantages. Bethanechol, for example, should be avoided by elderly patients while metoclopramide has a narrow therapeutic index, pronounced central nervous system (CNS) side effects and is known to stimulate prolactin release.
Patients suffering from other GI motility-related disorders such as delayed gastric emptying, diabetic gastroparesis, anorexia, gall bladder stasis, surgically induced adynamic ileus and chronic constipation (colonic inertia) may also benefit from treatment with prokinetic agents. In addition, prokinetic agents can aid in the placement of diagnostic and therapeutic instrumentation, such as during the insertion of enteral feeding tubes into the proximal small intestine.
Another, less common but very painful and disruptive GI motility disorder is chronic intestinal pseudoobstruction. Patients who are severely afflicted with this problem cannot tolerate oral feedings and require total parenteral nutrition. Metochlopramide and bethanecholine are also used in the treatment of this disorder but often with disappointing results. Prokinetic agents could not only be useful in alleviating the distress associated with this disorder, but also in severe cases could be used to facilitate treatment by decompression of the upper GI tract by nasogastric tubal aspiration. Increased gastric motility brought about by the use of a prokinetic agent has been shown to facilitate the placement of the necessary tubes into the intestine.
Macrocyclic lactone (macrolide) prokinetic agents are known. For example, J. S. Gidda et al., in European Patent Application No. 0349100, published Jan. 3, 1990, disclose 12-membered macrolides for use as gastrointestinal motility enhancers. S. Omura and Z. Itoh, in U.S. Pat. No. 4,677,097, issued Jun. 30, 1987, European Application No. 215,355, published Mar. 25, 1987, and European Application No. 213,617, published Mar. 11, 1987, disclose derivatives of erythromycins A, B, C and D which are useful as stimulants of digestive tract contractile motion. However, the compounds of these references are distinct from those of the present invention, in which novel lactam derivatives of the erythromycins are disclosed which possess an unexpected degree of prokinetic activity.